1. Field of the Invention
The present invention relates to a gate control circuit for turning on and off a voltage drive switching element such as an insulated gate semiconductor device of a MOS gate structure, for example, a MOS-FET, an IGBT (Insulated Gate Bipolar Transistor) and an IEGT (Injection Enhanced Gate Transistor).
2. Description of the Background
The insulated gate semiconductor device is used as a switching element of a power converter, for example an inverter which drives an induction motor, a frequency modulator and an uninterruptable power supply for supplying power to peripherals in case of a power failure.
Above all, the IGBT has been widely used in industrial equipment due to its voltage actuation characteristic and its capability of switching a relatively high current.
FIG. 1 shows a conventional gate control circuit for IGBT 1. In FIG. 1, IGBT 1 is an insulated gate semiconductor device having a collector terminal C, an emitter terminal E and a gate terminal G. A gate control circuit 100, shown by the chained line, is connected between the terminals G and E.
The gate control circuit 100 is composed of a first DC (Direct Current) power source 2A, a second DC power source 2B, a first switch 3A for applying a positive voltage to IGBT 1, a second switch 3B for applying a negative voltage to IGBT 1, and a gate resistor 4. A series circuit of the gate resistor 4, the first switch 3A and the first DC power source 2A is connected between the terminals G and E of IGBT 1. Further, a series circuit of the second DC power source 2B and the second switch 3B is connected between the junction of the terminal E and a negative terminal of the first DC power source 2A and the junction of the first switch 3A and the gate resistor 4. Furthermore, the terminal C and the terminal E are connected to a main circuit of a power converter (not shown).
IGBT 1 is turned on by applying a positive voltage across the terminals G and E, and is turned off by applying a negative voltage across the terminals G and E.
FIG. 2 is a timing chart showing a relationship between the voltage Vce and current Ic between a collector terminal C and an emitter terminal E of IGBT 1, and the voltage Vge and current Ig between a gate terminal G and an emitter terminal E of IGBT 1 at the time IGBT 1 turns off.
Upon turn off, the second switch 3B switches on in order to apply the negative voltage which is a driving voltage Vg. Then, the voltage Vge, called a mirror voltage, between the terminals G and E is maintained during turn off transition. The gate current Ig is almost constant during the transition. When the integration of the gate current Ig (electrical charge) reaches a certain amount of electrical charge, a current Ic flowing in IGBT 1 decreases and finally IGBT 1 turns off.
However, as shown in FIG. 2, there is a time delay to turn off IGBT 1 completely after switching on the second switch 3B because the mirror voltage has a positive value for a while (a mirror time). The mirror voltage, between the terminals G and E, caused by a mirror effect being proper to IGBT 1 remains until electrical charge stored in stray capacitance between the terminals G and E is completely discharged. This delay time has an adverse influence on efficiency.
One method to reduce the delay time is to lower the resistance of the gate resistor 4 and increase the gate current Ig. But this approach gives rise to a surge voltage between the terminals G and E, because the gate current Ig rises suddenly. Therefore, lowering the resistance of the gate resistor 4 results in increasing a switching loss of the IGBT 1. As a result, the resistance of the gate resistor 4 can not be changed easily.
Moreover, if a power converter is composed of a plurality of insulated gate semiconductor devices such as IGBT 1 respectively connected in serial or in parallel and the turn off delay times are different from each other, the power converter loses balance in the voltage or current applied to the insulated gate semiconductor devices. As a result, a high voltage or current is concentrated on one of the insulated gate semiconductor devices.